Introduction
It is well known that electromagnetic radiation, a fundamental energy permeating the entire universe, affects living organisms in a wide variety of ways. Depending on the frequency or wavelength of the radiation and on its intensity or power level, electromagnetic radiation, also known as EMR, may be beneficial or hazardous to living creatures.
Although radio waves and low power microwaves used in cell phones are largely considered benign, deep UV (ultraviolet light) and x-rays are known to be carcinogenetic and potentially life threatening to living creatures even at moderate doses. Still other frequencies, such as visible light, are beneficial and necessary to organisms, helping power earth's biosphere by enabling photosynthesis in plants and bacteria at the base of our planetary food chain. EMR has also become an indispensible tool used in radio frequency and microwave communication, and in the infrared portion of the spectrum for imaging and night vision. Unwanted electromagnetic radiation is often referred to as EMI or electromagnetic interference.
In electronics, meeting government established standards for the emission of electromagnetic noise and avoiding unwanted interference with other electrical devices is referred to as EMC, or electromagnetic compatibility. Other government standards apply to the maximum accepted power level emitted by a device (e.g. the brightness of a laser pointer or an industrial laser, or the maximum power level of a microwave oven or a microwave communications tower), and especially for any apparatus involving ionizing radiation such as alpha particle and x-ray sources or nuclear material.
Physicists today regards EMR as a spectral continuum (collectively the electromagnetic spectrum) describing a single type of energy based on the electromagnetic force in nature, varying by its frequency (alternatively by its wave length or wave number) and by its brightness, flux density or intensity. The electromagnetic spectrum 1 shown in FIG. 1 described (from left to right) in bands of decreasing wavelength (and increasing frequency) includes AC power distribution at the longest wavelengths (not shown), followed by radio, microwave, infrared, visible light, ultraviolet, x-rays, gamma-rays, and beyond that, cosmic rays (not shown).
Of particular importance to humans, the visible spectrum of light (or visible band 3) ranges from 750 nm to 400 nm varying monotonically in color from red to orange, yellow, green, blue, and to purple (as seen in a rainbow). The combination of these colors produces white light especially important for color vision. Visible light is also important in plants and algae for powering photosynthesis. Chloroplasts, organelles within plant cells, use chlorophyll to capture sunlight and to convert it into energy using the process of photosynthesis. Since the chlorophyll absorbs red, blue, and violet light, it makes plants appear green in color.
Light wavelengths in the band adjacent to visible light longer than 750 nm and shorter than 1 mm are referred to as the infrared band 2, with those closest to visible light referred to as “near” infrared or NIR and longer wavelengths as long infrared and far infrared. Long infrared light in the 8- to 15-μm range is used for infrared imaging 6 in medical and security applications.
Light wavelengths in the adjacent band shorter than 400 nm and longer than 10 nm are referred to as the ultraviolet band 4 with those closest to visible light referred to as near UV and the shortest wavelengths in the hand as extreme UV or deep UV. Just beyond the ultraviolet band, the X-ray band 5 comprises soft X-rays down to wavelengths of 0.1 nm and hard X-rays beyond that. Soft X-rays are used in security applications 7 for cargo and passenger inspection while shorter wavelength X-rays are used in X-ray crystallography, in radiography 8, and in computerized tomography or CT scans.
All EMR is the propagation of energy through space and in matter achieved through a time-varying electric field and a corresponding complementary magnetic field created through the movement or vibration of charged particles. Since the time-varying electric field induces a corresponding time-varying magnetic field, and conversely the time-varying magnetic field induces an electric field, EMR can travel without any medium, even in the vacuum of space. Its ability to penetrate matter depends on the absorption and scattering properties of the matter at each particular EMR wavelength.
Traveling in space or in matter, EMR is able to manifest itself as either a particle or a wave (but not both at the same time). When EMR manifests itself as a particle it is commonly referred to as a “photon”, while when it behaves in a wavelike manner it is often referred to as “light waves”. The term “light”, then, is used in two ways—in the general sense to mean any electromagnetic radiation in the spectrum, and in the specific case to mean only visible light and its spectral neighbors ultraviolet light and infrared light.
When EMR does come in contact with matter, it may be reflected, pass through the matter or be absorbed altogether, affecting the EMR and often changing the matter too. EMR's interaction with matter may manifest itself with particle like behavior governed by classical physics (first historically in the “Compton Effect”), as a wave exhibiting any combination of classical wave-like phenomena such as reflection, refraction, or interference, or by quantum mechanical effects such as quantized energy band transitions, molecular transformations, or quantum mechanical tunneling. Such interactions between matter and EMR have been harnessed for a large number of commercial, scientific, and medical applications.
Interactions of EMR and matter have been used extensively in scientific research, especially in imaging and analytics. X-ray diffraction allows for precise analysis of crystalline morphology and even played a crucial role in the discovery of DNA. EMR is also used extensively for medical imaging. Today, X-rays are routinely employed in radiography 8 to identify broken bones and dental cavities and in CT scans to identify tumors and tuberculosis. Imaging can also be performed using infrared light 6 often to analyze tissue where X-ray analysis is inconclusive or inconvenient. Advanced research also includes new hand held IR imaging devices that can identify carcinoma in tissue during surgery, particularly useful in identifying and removing cancerous cells in the border tissue during a mastectomy, avoiding the need for costly delays waiting for lab results and the repeat surgeries that result from the delayed lab analysis.
EMR also plays an important and growing therapeutic role in medicine. In some cases EMR is used to kill foreign or unhealthy cells while in other examples EMR is used to stimulate healing, promote immune response, reduce pain, and alleviate local inflammation.
Perhaps the best-known and oldest therapeutic use of EMR is radiation therapy 9, applied primarily in the treatment of cancer. Because such radiological protocols involve exposing a patient to ionizing radiation in the X-ray band 5, patients often suffer serious side effects from radiation poisoning that compromise the medical benefit by lowering a patient's quality of life. The goal of radiation therapy is to achieve targeted cellular destruction of the cancerous cells without damaging or killing a significant number of normal cells. Research continues in localizing the radiation to minimize the collateral damage to normal cells. In many cases, however, it is simply a matter of statistics—a race to destroy most of the cancer before the treatment kills the patient.
Another therapeutic protocol of EMR using less energetic photons than X-rays employs ultraviolet light 4 to remove unwanted antigens and bacteria from the skin. One common example of UV phototherapy is the anti-bacterial use of ultraviolet light 11 locally applied by dermatologists to treat skins rashes and irritations, in essence to “sunburn” whatever may be present on a patient's skin causing a rash or itching. Its use is often indicated even when the actual cause of an irritation cannot be confirmed.
In general, to destroy or otherwise inhibit the growth of mutated, pre-cancerous, cancerous, or un-normal cells requires the concentration of EMR into a narrow bandwidth and a focused area to avoid damaging healthy cells. Applying a focused beam to treat large areas is problematic—especially once metastasis has commenced. Research in using beams of focused energy to perform targeted cellular destruction spans the spectrum from the far infrared to hard x-rays.
EMR is also used for therapies not targeting cellular destruction, but instead to promote natural healing processes within the body. In thermotherapy 10, long wave infrared heating either through lamps or LEDs is used to raise a patient's body temperature to that similar to temperatures experienced from mild exercise. Research shows thermotherapy benefits patients suffering from severe cardiovascular disease unable to exercise by improving cardiovascular flow over large areas and volumes of the circulatory system.
Another EMR therapy, herein referred to as photo-optical stimulation 12, is used to counter depression and anxiety in people by visually stimulating a patient's eyes with artificial colored or white light (essentially emulating sunlight or portions thereof) to enhance their mood and provide a sense of wellbeing. Such photo-optical stimulation treatments are especially important for residents of Polar Regions where extended periods of darkness prevail for the majority of winter days, and where alcoholism and suicide rates greatly exceed that of the general population globally. Photo-optical stimulation treatments have also been used to counter severe jet lag and to restore and help regulate sleeping patterns.
Photobiomodulation
Referring again to FIG. 1, phototherapy 14, the emerging medical field to which the apparatus of this disclosure relates, herein is broadly defined as the therapeutic application of light to beneficially affect cells and tissue through photobiomodulation and the photoexcitation of No-molecules.
Photobiomodulation, the electrochemical response of cells and tissue to direct illumination by ultraviolet, visible and infrared light, represents a physical mechanism by which energy is imparted directly into a cell by photons to produce any number of photobiological responses, including forming ATP (a molecular source of energy), accelerating intracellular and intercellular chemical reactions, stimulating DNA transcription and RNA translation (protein synthesis), increasing intracellular catalyst concentrations, and beneficially affecting the cell and its host. Depending on the wavelength of the impinging light, photobiomodulation has been observed in virtually every living creature on earth from bacteria to complex organisms, including animals, mammals, and even HOMO sapiens. Since, in many cases, photobiomodulation stimulates the formation of ATP and initiates protein synthesis, it means (at the right frequencies of light) photosynthesis occurs not only in bacteria and in plants, but also in animals.
One of the early observations of photobiomodulation was reported in 1967 by a researcher studying the biological effects of infrared laser light on animals. Convinced the laser light caused cancer, the researcher shaved mice then irradiated half of the population with low-level infrared light only to discover not only did the mice not contract cancer, but much to the researcher's surprise, the irradiated mice's hair grew back at a greatly accelerated pace.
The report was largely forgotten until later and quite accidentally) photobiomodulation was essentially re-discovered by NASA during the space shuttle program, observing astronauts exposed to infrared light from a plant grow lamp healed normally when those from prior missions absent the grow lamp did not heal normally. Further research identified mice poisoned with methanol and treated with infrared light retained most of their vision while the control group went completely blind. The study identified methanol molecules which normally attach to the adenosine triphosphate (ATP) chemical bonding sites in the optical nerve (and essentially starve the nerve tissue for energy) are dislodged and replaced by ATP generated by photobiomodulation of infrared light. The significance of this study was its early identification of ATP and its role in the photobiological process.
Molecular studies on bacteria and plankton subsequently revealed that light (especially in the visible red and near infrared portion of the spectrum 13) is absorbed directly by a biomolecule cytochrome C oxidase (CCO). The CCO molecule acts as an intracellular battery charger converting adenosine monophosphate (AMP) into adenosine diphosphate (ADP), and then into adenosine triphosphate (ATP). If metaphorically. CCO is the battery charger, then ATP is a cell's battery—the battery powering all the other electrochemical reactions within the cell itself.
Studies on mice and human patients found photobiomodulation of injured tissue or organs resulted in cellular repair, tissue regrowth and accelerated healing, improved immune response, reduced tissue inflammation, lower secondary infection risk, and faster recovery from injury or illness. In human trials, patients also reported reduced pain and improved health following only a few phototherapy treatments. In patients suffering from peripheral neuropathy, i.e. the death of nerves in limbs common in diabetes sufferers, partial recovery of their sense of touch was also noted. Numerous refereed professional journals (e.g. journal of Lasers in Medical Science) and various textbooks are available today summarizing the discovery of photobiomodulation and its prospects for therapeutic treatments, i.e. for “phototherapy” [Charles T. McGee, “Healing Energies of Heat and Light” Medipress, Coeur d'Alene, 2000].
To date, photobiomodulation and its potential therapeutic effects have been studied for over four decades. Despite exciting, almost unbelievable early results, decades passed without doctors or scientists finding a practical or commercially viable means by which to bring phototherapy to market, due to limitations in understanding and more so in the technology of the day.
As shown in FIG. 1, phototherapy may be performed using either incoherent light 17 or by lasers 15. Initially, incoherent light from the sun and later from lamps was used, in part because of its broad-spectrum 17A and its ability to cover a large area 17B. Lamps however were found to be unwieldy, consuming large amounts of power and producing more heat than light. Subjects complained of overheating and researchers suffered burns from handling the hot incandescent bulbs. Other studies especially in the USSR, focused on applying ceramic infrared lamps adapted from industrial ovens and heaters, into phototherapy. These efforts concentrating primarily in the long wavelength portion of the spectrum include both beneficial phototherapy as well as targeted cellular destruction. Later, scientists also attempted treatments using gas discharge lamps ands noble gasses but were limited by their inability to control the radiated wavelengths.
Research then turned to lasers 15 operating at much cooler temperatures than bulbs and beneficially producing more light than heat. At first it was thought that coherent light would impart an added advantage in penetration depth and treatment efficacy but it was soon discovered that the small spot size 16B of a laser made the treatment of large areas such as a whole organ or large muscle problematic. Further studies also revealed that optical coherence was almost immediately lost from scattering in the top layers of the skin anyway, so that deep penetrating light was not coherent even when emanating from a coherent source. Moreover, gas and dye lasers were costly, fragile, large, heavy, power hungry, and inconvenient to transport. Other studies revealed that too narrow of a frequency-spectrum 16B typical of laser light might adversely reduce phototherapy treatment efficacy.
While the advent of the semiconductor laser diode helped lower the potential cost of laser phototherapy, the small spot size of laser diodes combined with a characteristically narrow bandwidth remain problematic. Moreover, laser medical devices continue to pose a potential safety risk to both patients and clinicians and require strict compliance with an ever-changing set of governmental regulations. As such, the broad scale commercial deployment of phototherapy devices based on the use of laser diodes still face numerous challenges.
In contrast, the recent commercial availability of relatively low-cost bright LEDs offers a more promising means by which to engineer a practical phototherapy device. Unlike laser diodes, LEDs are rapidly emerging as the preferred source of light to be employed in a virtually unlimited range of applications. Today LED lighting exclusively provides the backlight and camera flash in virtually every mobile phone and smartphone sold. LEDs also facilitate backlighting for the newest generation LCD HDTVs offering “green” (i.e. energy efficient) operation and enhanced image contrast. Since 2010, the LED began expanding into general lighting applications including automobile headlamps, tail lamps and cabin lighting; into streetlights, and even into commercial and residential lamps replacing inefficient incandescent bulbs and obsoleting hazardous mercury-contaminated compact fluorescent lamps (CFLs).
With its ubiquitous use driving high production volumes, the resulting economy of scale benefits, supplier competition, and new technology continue to drive LED costs lower, further enabling the LED's competitive advantage in the global marketplace. Moreover, the LED's exceptional safety record prompted the United States government to further relax LED safety regulations, distinguishing the LED as a separate and distinct category from lasers and laser diodes, authorizing unrestricted use provided an LED's power output does not exceed the FDA's guideline of 300 mW/cm2.
LED Flashlight and Torchlights
One requirement for effective LED phototherapy is an electro-optical design capable of maintaining a consistent LED current for extended treatment durations, e.g. at least 60 minutes or longer. Low-cost LED driver circuitry and designs used in flashlights, torchlights, and many phone backlights, however, lack the necessary features and capability to adequately control LED brightness over such extended durations, or to distribute light uniformly. They also lack the ability to perform a number of important functions useful in customizing treatments to specific medical conditions, e.g. by modulating LED excitation to vary its operating frequency. FIG. 2, for example, illustrates a conventional low-cost LED driver where four series connected batteries 20a through 20d power one or more parallel strings of LEDs 26a through 26n with each string comprising “m” series connected LEDs.
Counter 23a and inverter 23b, collectively as digital controller 23, digitally oscillate at a fixed clock frequency by repeatedly switching MOSFETs 27a through 27n on and off, in turn toggling on and off LED currents ILED1 through ILEDn in the LED strings 26a through 26n. By varying the pulse width and corresponding duty factor D of the LED current conduction time by controller 23, strobe operation of the LEDs is able to facilitate fixed frequency PWM brightness control.
Unfortunately, a fundamental problem with the circuit as shown is that it powers the LED strings with a voltage source, not current sources. LEDs prefer, i.e. behave better, being driven by constant current sources. Voltage source drive cannot balance the current evenly among the LED strings 26a through 26n because the LED forward voltages do not match one another, varying stochastically with manufacturing and also varying dynamically with operating current and brightness. Series ballasting resistors 24a through 24n are included in an attempt to balance the current more evenly among the LED strings but they still do not guarantee matching of current or LED brightness.
Unlike current source drive, using a voltage source and a resistor allows the LED currents and brightness to change with the power supply voltage, i.e. with the decay in the battery voltage. As shown in FIG. 3, as the battery voltage Vbatt 30 declines, the potential difference between it and the LED string voltage VLED 31 declines in proportion. Since the LED current in any given string is given byILED=(Vbatt−VLED)/R then any change in Vbatt over time will manifest itself as a time dependent change in LED brightness. When Vbatt approaches VLED during the battery's decay, the LEDs eventually turn off and cease to illuminate. LEI) strings with a higher forward voltage will dim faster and turn off sooner than lower forward voltage strings causing inconsistent LED brightness over time and poor luminous uniformity as well.
Returning to FIG. 2, in order to prevent flicker resulting from switching noise and current transients among channels, capacitors 25a through 25n are added for filtering. Low dropout (LDO) linear regulator 21 and filter capacitor Creg 22 are also included to provide a consistent voltage Vdriver to supply digital controller 23 despite variations in the output voltage Vbatt of battery pack 20 during discharge and with LED current transients. While LDO 21 could also be used to power LED strings 24a through 24n, the extra voltage drop across the LDO adversely affects the device by diminishing the peak LED brightness, increasing power dissipation, and resulting in the LEDs shutting off sooner than they would otherwise.
In the design, the maximum number of series connected LEDs “m” depends on the LED forward voltage, the battery chemistry and the resulting battery voltage Vbatt. If battery 20 comprises a rechargeable LiIon chemistry each cell exhibits a nominal voltage of 3.6V and Vbatt=14.4V. If red LEDs are used with a forward voltage of 1.6V each, then “m” may be chosen for 8 or 9 LEDs. If infrared LEDs are employed, each LED has a forward drop of 2.2V so that “m” is limited to 5 or 6 series LEDs. If strings of red LEDs are alternated the string-to-string voltage and current mismatch problem will be further exacerbated. The LED torchlight drive shown simply cannot accommodate such mismatches in voltage and current, and therefore mixing LED types is problematic.
In consumer devices, however, alkaline batteries are far more common than expensive rechargeable LiIon cells. In such cases, each cell starts at approximately 1.5V but gradually decays to 1V per cell, with the battery voltage ranging from slightly over 6V and decaying to 4V and beyond. During the discharge, the LED brightness will decline constantly during use. At 4.2V, the LEDs no longer illuminate and the batteries must be replaced to continue use.
Attempt to regulate the LED voltage by introducing a voltage regulator between battery pack 20 and the LED strings only makes the problem worse because the converter itself, consumes power further shortening battery life. If the converter is another LDO similar to LDO 21, the voltage drop across the LDO actually shortens the battery life. If an expensive boost converter is employed producing a fixed voltage higher than Vbatt, a new problem occurs. Since the forward drop across LED strings varies stochastically with manufacturing, at higher operating voltages and correspondingly, with a larger numbers of series connected LEDs, variations in LED voltage can be substantial, especially during the entire manufacturing life span of a product.
To insure every LED string always has sufficient voltage to illuminate at specified current the fixed voltage output from a boost converter must exceed the highest string voltage expected in the course of manufacturing. This design approach naturally results in a “high” supply voltage—one higher than needed for normal units having LED voltages near the statistical mean. The unused excess voltage produces heat in the driver circuit in resistors 24a through 24n and in MOSFETs 27a through 27n, lowering efficiency and shortening battery life. In LED strings having LED voltages below the mean, i.e. at the low end of the distribution, the extra voltage can become excessive, even causing overheating in the drive circuit. If the LED current is also varied, the problem becomes further exacerbated because the LED string voltage is also a function of operating current.
Mechanical design represents another significant limitation of LED torchlight designs. FIG. 4A illustrates an artist's conceptualization of LED torchlight 35 typical to this genre of phototherapy products. The torchlight or “wand” has a handle portion and an LED portion comprising an array of LEDs 35a. The LED array is stiff and inflexible with the LEDs essentially coplanar mounted on a circuit board housed within wand 35. Other versions embed the LEDs within a hairbrush.
The first problem of this design is the practical consideration of treatment time. Phototherapy achieves photobiomodulation by introducing a sufficient number of photons into tissue to change the electrochemistry of cells in the treated tissue. The maximum rate photons can be introduced into tissue is practically limited to the highest LED brightness to avoid skin burns and to comply with government regulations. Considering these aspects, minimum treatment times are in the range of 20 minutes to 60 minutes depending on the tissue being treated. Times shorter than the prescribed amount are completely ineffective, analogous to plugging a phone into a battery charger for only a couple of minutes. In such a short duration, the electrochemistry of a cell, like a battery, does not normalize and energy is not absorbed in any beneficial way.
Holding a wand on your skin in one specific place for 60 minutes without moving it is just not possible. Any movement changes the treatment conditions. As shown in the leftmost example of FIG. 4A, when LED wand 35 is held above the skin at a distance, photons spread over a large area but are primarily absorbed by the outer epithelial layers of skins, barely penetrating into the subdermal tissue 37. Slightly moving the wand 35 as in the middle graphic results in an entirely different therapy volume, being smaller in area and deeper in penetration. The right most case shows close proximity treatment, illuminating a surface area no larger than that of the LED array 35a but penetrating deep into subdermal layers 37 To cover a large area at close proximity, a patient would be required to hold the wand in a fixed position for long durations multiple times until all the area was treated. Such an exhaustive procedure is simply not practical especially for patients suffering duress from illness or pain. Likewise, no clinic can afford to hire a nurse to sit with a patient during the entire treatment just to hold LED wand 35 in place.
Expanding the size of wand 35 doesn't solve the area problem either. Making the LED area 35a larger further exacerbates the issue of achieving uniform penetration depth over large areas, especially because most treatment areas on a patients involve curved surfaces, e.g. a leg, arm, neck, side, etc. As shown in FIG. 4B, illuminating curved body surfaces with a stiff planar LED array results in a continuously variation distance between LED and skin, resulting in a large variation in photon penetration depth 38. As illustrated, the portion of planar LED array closest to the skin in the center of the array will penetrate deeper than the edges.
The tighter the curvature of the treated body part, the worse the uniformity problem becomes, with arms and legs and fingers suffering the poorest illumination uniformity.
Combining the low cost LED torchlight drive circuitry with a flat wand or flashlight illuminator shape renders such consumer oriented completely impractical to use and ineffectual in their result. Given these severe design issues, such consumer “gadgets” cannot be considered as real phototherapeutic devices or as prior art apparatus or method for achieving photobiomodulation or delivering phototherapeutic treatments.
HDTV Backlight Drivers
State-of-the-art for the electronic drive of LED arrays today is best exemplified by integrated circuits (IC) for LED backlighting of large LCD (liquid crystal display) panels, especially those used in LED backlit HDTVs (high definition televisions). While the end application, design, operation, and software programming of these systems have been specifically designed and created for driving white LEDs in LED backlit HDTVs, the hardware and IC implementation of such systems is vastly better than the torchlight driver circuitry used in present day phototherapy devices and consumer gadgetry.
FIG. 5A illustrates the basic elements of a LED backlit HDTV comprising a liquid crystal display (LCD) panel 42, a color filter 43, and a LED backlight including an array of white LEDs 41 and LED backlight driver IC 45. TV viewers 44 observe color 2D or 3D images on the LCD panel by seeing light produced by the array of white LEDs 41 penetrating some fraction of the pixels, i.e. picture elements, in liquid crystal display panel 42 and passing through color filter 43. LCD panel 42 acts like an adjustable window-blind offering 256 or more “grey scale” levels of light transmissivity ranging from black (opaque) to full brightness (transparent) and every intermediate brightness. The combination of black, white, and grey pixels forms two-dimensional (2D) images on the display as seen simultaneously by both left and right eyes of observer 44.
Transmitted light passes through color filter 43 with the light from any given LCD pixel passing only through one of three colors—red, green, or blue. Because white light emanating from white LED 41 contains all the colors of the rainbow, color filter 43 is able to filter each pixel into one color only, either red, blue or green, For example, a red color filter actually absorbs all the colors except red light, removing blue green, violet, etc., to give the light a red color.
The combination of all three colored pixels, one red, one blue and one green, can then be used to recreate images with virtually any color in millions of shades and brightness levels. Such a display is referred to as a RGB color LCD. The data sent to each red, green, and blue pixel are processed by a video processor IC and set to a combination of row and column drivers, managed by a complex digital video timing controller. A new picture is loaded and rescanned at a fixed period known as the vertical synchronization or Vsync pulse which typically occurs a rate of 60 Hz in older TV models and at 120 Hz or 240 Hz in today's newest high performance HDTVs.
Synchronized to the LCD image and to the Vsync pulse, LED backlight driver 45 controls the brightness of the array of white LEDs 41 using pulse width modulation (PWM) brightness control operating a fixed clock frequency and updated once every Vsync pulse. The backlight may be uniform in brightness operating at a single duty factor D and adjustable in brightness for the entire display. Uniform backlight brightness of an LCD backlight is referred to a global dimming control. Alternatively, the backlight may be broken into tiles or segments with each tile illuminated to the proper brightness corresponding to the image in that portion of LCD panel 42 located directly above the backlight tile. By varying the LED brightness in conjunction with the image, e.g. where dark portions of the image are illuminated by a dimmer LED backlight, power is saved and image contrast enhanced. Using local dimming, blacks look blacker, and bright images look brighter, for the first time enabling the means for “mega-contrast” performance in LCDs comparable to that of power hungry plasma displays.
Local dimming in LED backlit LCDs is achieved by “current control” of LED string currents using ground-connected current sources, commonly known as “current sinks” as shown in FIG. 5B. These current sinks 56a through 56n individually respectively control the current in LED strings 57a through 57n. Each current sink includes feedback (schematically represented by a current sense loop and analog input to each dependent current source) to dynamically adjust gate drive of the transistors comprising said current sink in order to maintain a preprogrammed value of sink current independent of voltage. Each current sink 56a through 56n is toggled on and off by digital signals 58a through 58n respectively.
These digital signals are output from LED driver circuit 55 comprising LED driver ASIC 56 a set of digital buffers needed to drive the line capacitance of signals 58a through 58n distributed across the LED backlight board. Switched at a fixed frequency, the digital signals 58a through 58n each vary independently in duty factor D to adjust the corresponding current and brightness of each LED string 57a through 57n. Global dimming and overall brightness control is performed by operating each LED string at the same duty factor D. Operating each string at its own unique duty factor D1 through Dn performs local dimming in response to instructions received from video scalar IC 54 communicating to LED driver 55 through digital SPI bus 59a. In some cases the video scalar IC 54 communicates to a microcontroller (not shown), interpreting and in turn instructing LED driver 55 regarding the proper drive for each LED string for local dimming.
As shown, switch mode power supply 52 and filter capacitor 53c power all the LED strings at a regulated voltage +VLED. White LEDs are generally constructed using wide bandgap materials to produce blue light. The blue light is subsequently converted into white light by phosphor in the LED lens. Because white LEDs employ wide bandgap materials, the voltage drop across white LEDs is quite high, typically 3.5V to 4V per LED. This means the output of SMPS 52 is often high voltage, ranging from 60V when in, the number of series connected LEDs, is below 15, to over 200V for larger values of m.
The voltage rating of capacitor 53c should be scaled accordingly with its capacitance value Creg3, sufficient to stabilize the regulator's control loop and adequate to support the worst-case LED transients without dropping out of regulation. Individual ripple filtering capacitors on each LED string (like those shown in FIG. 2) are not needed in this design because current sinks 56a through 56n maintain the individual LED currents ILED1 through ILEDn despite fluctuations in voltage arising from transient voltage drops or string-to-string voltage mismatch. In battery powered applications such as a notebook computers Vin is typically in the range of 15 to 20V, SMPS 52 typically comprises a boost converter and Vlogic and Vdriver are regulated using LDO linear regulators 50 and 51.
In monitors and HDTVs, the input voltage is typically the AC-mains, either 110 VAC or 220 VAC, and SMPS 52 is generally an isolated flyback converter. In such cases SMPS 52 often includes a second output, typically 24 VDC or 12 VDC, sued to power LDOs 50 and 51.
As shown in FIG. 6A, PWM brightness control in 2D HDTVs if performed using a fixed frequency clock pulse to generate a programmable duty-factor-controlled LED (output) current waveform. A microcontroller, video processor IC, or timing generator IC generates a vertical sync signal Vsync 60 having a period Tsync typically corresponding to a frequency of 60 Hz, 120 Hz, or 240 Hz. The lowest of these frequencies, i.e. 60 Hz, was chosen historically to be the lowest frequency that was sufficiently fast that the human eye could not see the screen image change or flicker. More recently double or quadruple Vsync frequencies have been used to reduce image blur and to facilitate 3D displays. The Vsync pulse acts as the main clock in an LCD or HDTV as it is used in a variety of functions including the instruction to load video data from the video processor into the LCD column drivers, to advance the LCD scan by one row, and to load information into the LED backlight driver control registers.
In an LED backlit HDTV offering brightness control, a second clock 61 is generated and synchronized to the Vsync clock 60 but operating at a higher frequency. For example in a HDTV offering 4096 levels or 12-bits of dimming control, the second clock, sometimes referred to as a grey scale clock or GSK, runs at a frequency fθ that is 4096 times faster than the Vsync pulse rate, i.e. having a grey scale clock period of Tθ=Tsync/4096. By employing programmable counters in an LED driver IC, the average LED brightness can be varied from 0% to 100% in 4096 steps either locally or globally, each step representing approximately 0.0244% variation in backlight brightness.
This backlight brightness control and dimming feature can be implemented without the need to change LED conduction current. In the graph for Current Reference 62, the value of LED current in any channel set by the precision Current Reference 62 remains constant at a user programmable value 62a equal to Iref throughout. Instead of changing currents, the duty factor D is varied dynamically to adjust backlight brightness. Referring again to FIG. 6A, the LED on-time is initially operated at an specific on-time ton1 resulting in a 66% duty factor shown by curve 65a whereD1=ton1/Tsync=66%meaning during each Tsync period prior to time t1, LED (Output) Current 63 operates two-thirds of the time in an on-state 64a conducting a current equal to αIref and one-third of the time 64b in an off-state at zero current. Described in terms of the programmable counter and Clock 61 switching at frequency fθ=4096/Tsync, the on-time and off-time for 66% duty factor operation is 2730 clock pulses and 1366 clock pulses respectively. The average LED current and therefore LED brightness shown by curve 65a represents a level 66% that of the pulsed current value αIref.
Between time t1 and time t2, the brightness control changes to a duty factor of 50% as shown by average value 65b, so thatD2=ton2/Tsync=50%and where ton2 digitally represents 2048 clock pulses, or half the number of the period's 4096 pulses. Similarly between time t2 and time t3, is the brightness control increases to a duty factor of 75% as shown by average value 65c, whereD3=ton3/Tsync=75%and where ton3 digitally represents 3072 clock pukes, or three-quarters of the period's 4096 pulses. After time t3 the average duty factor 65d drops to only 12% or 491 pulses per period. Backlight operation at such low duty factors typifies sleep mode operation where a display is dimmed dramatically to reduce power consumption, save battery life, or improve a display's green power rating.
FIG. 6B shows the waveforms for the same display changing operation from 2D mode into 3D mode. Three dimensional image display in HDTVs, also known as 3D mode, involves alternately displaying two images, one for the left eye, the other for the right eye, and switching the images at a sufficiently high rate that the eye cannot see the alternating images. The left and right images are separated using glasses worn by the viewer that only allow the left eye to see the left eye image and only allow the right eye to see the right eye image. This may be accomplished using passive glasses comprising two different polarizing filters and by changing the polarization of the display image in alternating fashion to direct the image to the corresponding eye. Alternatively, active glasses comprising LCD shutters synchronized to the display images may be used to control which eye sees which image.
In any event, because only one eye sees the display's image at a time, the duration by which the image is displayed must be half the time of that in 2D mode. To avoid the perception of flicker the image must be scanned at twice the normal Vsync. For example if a HDTV normally operates a 60 Hz Vsync rate, then in 3D mode the display and the backlight must operate at 120 Hz. If a HDTV normally operates at 120 Hz in 2D mode, in 3D mode the Vsync rate is doubled to 240 Hz.
As shown prior to time t7, the Vsync pulse occurs at a fixed rate with a period Tsync for normal 2D mode operation, after which, the Vsync rate doubles to a pulse with period Tsync/2. At time t7, Grey scale Clock 61 also doubles in frequency from fθ (2D mode)=4096/Tsync to a rate fθ (2D mode)=4096/Tsync. Although the clock rate doubles, since the programmable counter relies on the clock, the duty factor stays constant. For example, between time t6 and time t7, the brightness control has a duty factor of 50% as shown by average value 65f, wherebyD7=ton7/Tsync=50%
After the frequency doubles at the onset of 3D operation, ton8 is reduced to half the value of ton7, i.e.ton8=ton7/2
But likewise, Tsync3D is reduced to Tsync/2 so thatD8=ton8/Tsync3D=(ton7/2)/(Tsync/2)=D7=50%
So changing the Vsync frequency has no bearing on the PWM duty factor or PWM brightness, as shown by duty factor curves 65f and 65g. But because in 3D mode only eye is seeing the display image at a time, the human mind perceives the brightness as half that of normal brightness. To compensate for this effect, the brightness of the LED backlight must doubled in 3D mode from αIref to a value of 2αIref. In other words, the brightness of the LED pulses doubles in brightness in 3D mode but since only one eye sees them at a time, there appears to be no change of brightness compared to 2D mode.
A TV backlight driver IC capable of performing all these operations is illustrated schematically in FIG. 7 comprising channel drivers 69a through 69n and control section 69z (collectively as LED driver 69) driving LED strings 57a through 57n powered by SMPS 52. In this system, video information from video scalar IC 54 is transferred via SPI bus 59a to microcontroller 67. Microcontroller 67 interprets this video information and passes it to the control section 69z of LED driver IC 69, specifically via SPI bus interlace 59b. The SPI bus then distributes the information to decoders 74a through 74n using digital bus 73 which instructs the individual channel drivers on drive conditions including timing and biasing. For high speed data transmission with a minimal number of interconnections, digital bus 73 represents some combination of serial and parallel communication. Since the bus is dedicated to the LED driver, such a bus may conform to its own defined standards and is not subject to complying with any pre-established protocol as SPI bus 59a and 59b are.
This digital information from digital bus 73, once decoded by decoders 74a through 74n, is next passed to digital data registers, i.e. data latches, present within each individual channel driver 69a through 69n. In the schematic of FIG. 7, the decoded data includes a 12 bit word defining up to 4096 increments in duty factor D, for brightness control, a 12 bit word defining up to 4096 increments in phase delay φ used to compensate for propagation delays across a panel and to minimize power supply inrush currents, and a 8 bit word Dot for setting the LED currents used in current calibration to improve backlight uniformity (i.e. dot correction) and used to switch between 2D and 3D display modes.
For example, synchronous to each vertical sync pulse on Vsync line 60, decoder 74a loads a 12-bit word into D register 75a, a 12-bit word into φ register 76a, and a 8-bit word into Dot register 77a contained within individual LED drive channel 69a. In similar fashion and synchronous to Vsync pulse on line 60, decoder 74b loads a 12-bit word into D register 75b, a 12-bit word into phase delay φ register 76b, and a 8-bit word into Dot register 77b contained within individual LED drive channel 69b. The same process occurs simultaneously for all n channels, i.e. from channel drivers 69a through 69n. 
Once the data from decoder 74a is loaded in duty factor D register 75a and phase delay φ register 76a, counter 78a begins to count pulses present on Clock fθ line 61, the output of counter 78a determined the timing of precision gate bias circuit 70a to toggle current sink MOSFET 71a on and off. By controlling the timing of conduction of current ILED1 flowing in LED string 57a including its duty factor D, i.e. its on time each Vsync period, the brightness of LED 57a is precisely controlled. The bit data loaded into Dot register 77a is simultaneously interpreted by D/A converter 79a to set the reference current αIref feeding precision gate bias circuit 70a. This reference current sets the analog magnitude of LED current ILED1 flowing in MOSFET 71a and in LED string 57a whenever the particular channel is toggled on and conducting. It has no bearing of the MOSFET's current when counter 78a toggles the particular 69a channel off. The same process occurs simultaneously for all n channels, i.e. from channel drivers 69a through 69n. 
The value of reference current αIref is set in any given channel driver in two ways. Firstly the value of Iref is set by a precision resistor Rset present in each channel driver 69a through 69n. A precision trimmed voltage reference Vref present within LED driver IC 69 (but not shown) is converted into the precision reference current Iref by the value of the resistor Rset such that Iref=Vref/Rset. The resistor Rset may be integrated provided that it is trimmed for absolute accuracy during manufacturing, or may comprise a discrete precision resistor, one per channel, externally connected to each channel of LED driver IC 69. While the value of Rset could conceivably be varied from channel-to-channel, it is generally preferable to use precisely the same value of Rset in every channel to maximize the channel-to-channel matching and to vary the channel reference current through the digitally controlled value of the parameter α, as determined by the digital value stored in the Dot register of every channel.
For example in channel driver 69a, the 8-bit word stored in Dot register is converted into one of 256 levels for the multiplier α, allowing the current in MOSFET 71a and in LED string 57a to be set anywhere from 0% to 100% ·Iref in 256 steps, i.e. in increments of 0.39% per step whenever MOSFET 71a is on and conducting. The same operation occurs in all channel drivers 69a through 69n, enabling digital control of LED current in every LED string 57a through 57n via digital bus 73.
It should be noted that in an LED backlit HDTV it is preferable to change LED brightness using the digital PWM dimming method and counters 78a through 78n than it is to use Dot registers 77a through 77n changing the corresponding value of αIref in each channel, primarily because the color temperature of white LEDs is a function of current. Running the LED strings across the display at dramatically different currents can adversely result in color aberrations in the display image.
To maintain the proper current in every LED string including the LED string with the highest forward drop, current sense feedback circuits CSFB 72a through 72n have been included to determine in real time which string exhibits the highest voltage and to use that information as feedback to SMPS 52 to set its output +VLED to a voltage just slightly higher than the LED string with the highest forward voltage.
The CSFB circuits are connected in daisy chain fashion, i.e. in series “head to toe”, with the input 73b into CSFB circuit 72a coming from the output of CSFB circuit 72b, the input 73c into CSFB circuit 72b coming from the output of CSFB circuit 72c (not shown), and so on. Each CSFB circuit passes the lower of its input voltage or the voltage on the drain of the corresponding current sink MOSFET in the channel to its output, until the last CSFB circuit 72a has an output 73a representing the lowest drain voltage in the IC (and hence the highest forward voltage LED string) provided as the CSFB feedback signal to SMPS 52. The first CSFB circuit in the string CSFB 72n must have its input tied to the highest convenient voltage, e.g. Vlogic or Vdriver.
Since, however, all the LED strings in a LCD backlight for color HDTVs are white LEDs, probably from the same manufacturer and even the same production batches, the variation in forward voltage of the LED strings primarily results from natural stochastic variability in the LEDs' manufacture, not from functional differences in the types of LEDs being driven.